Liquid crystals (LCs) have assumed their place as one of the most important materials of the information age. LC displays (LCDs) play a significant role in our everyday life; from handheld personal devices to professional applications and large-panel LCD TVs. LCs are liquids possessing long-range orientational ordering, lacking in most instances (phases) long-range positional ordering of the constituent molecules. In the case of the one-dimensionally ordered fluid nematic phase used in most display applications, intrinsic elastic interactions align the LC molecules along some preferred direction (director), which in most cases forms the optical axis of the material. Typically, nematic LCs are used in thin films, sandwiched between two glass substrates featuring transparent electrodes (usually indium tin oxide, ITO). These substrates are covered with so-called alignment layers, whose main role is to define the boundary conditions of the director to ensure uniform distribution of the optical axis is the entire LC thin film. These predominant boundary conditions are referred to as “homogeneous” (director lies in the plane of the thin film; usually with a small pre-tilt), “homeotropic” (director is normal to the plane of the thin film) or, less frequently, intermediate “tilted”.
Alignment layers commonly feature some type of anisotropy that induces a preferred orientation for the LC director on the surface. Unidirectionally rubbed polyimides are the most widely used alignment layers, providing stable alignment of nematic and smectic LCs for various display modes. However, this method also has numerous disadvantages, such as polymer debris resulting from the rubbing with a velvet cloth (using rubbing machines) and inhomogeneous, site-dependent contrast ratios in the final display, which can only be avoided by careful monitoring of the manufacturing conditions in clean rooms, see J. van Haaren, Nature 2001, 411, 29. Ion-beam deposition or plasma bombardment of thin polymer, SiNx, diamond-like carbon, or other thin films deposited on substrates are studied as well, see for example: (a) K. D. Harris, A. C. van Popta, J. C. Sit, D. J. Broer, M. J. Brett, Adv. Funct. Mater. 2008, 18, 2147; (b) Y. H. Kim, H. G. Park, B. Y. Oh, B. Y. Kim, K. K. Paek, D. S. Seo, J. Electrochem. Soc. 2008, 155, J371; (c) G. Hegde, O. Yaroshchuk, R. Kravchuk, A. Murauski, V. Chigrinov, H. S. Kwok, J. Soc. Inf. Display 2008, 16, 1075, and of these techniques, the glancing angle ion beam bombardment of diamond-like carbon used for the manufacturing of smaller LCD panels by IBM, see P. Chaudhari, J. Lacey, J. Doyle, E. Galligan, S. C. A. Lien, A. Callegari, G. Hougham, N. D. Lang, P. S. Andry, R. John, K. H. Yang, M. H. Lu, C. Cai, J. Speidell, S. Purushothaman, J. Ritsko, M. Samant, J. Stohr, Y. Nakagawa, Y. Katoh, Y. Saitoh, K. Sakai, H. Satoh, S. Odahara, H. Nakano, J. Nakagaki, Y. Shiota, Nature 2001, 411, 56.
Photoalignment and obliquely evaporated inorganic materials are alternative techniques that are utilized as well, for a recent review, see: O. Yaroshchuk, Y. Reznikov, J. Mater. Chem. 2012, 22, 286; for a review summarizing work up to 2000: K. Ichimura, Chem. Rev. 2000, 100, 1847; for representative examples, see: (a) J. Hoogboom, M. Behdani, J. A. A. W. Elemans, M. A. C. Devillers, R. de Gelder, A. E. Rowan, T. Rasing, R. J. M. Nolte, Angew. Chem., Int. Ed. 2003, 42, 1812; (b) J. Hoogboom, P. M. L. Garcia, M. B. J. Otten, J. A. A. W. Elemans, J. Sly, S. V. Lazarenko, T. Rasing, A. E. Rowan, R. J. M. Nolte, J. Am. Chem. Soc. 2005, 127, 11047; (c) Y. Morikawa, S. Nagano, K. Watanabe, K. Kamata, T. Iyoda, T. Seki, Adv. Mater. 2006, 18, 883; (d) O. Klikovska, L. M. Goldenberg, J. Stumpe, Chem. Mater. 2007, 19, 3343; (e) L. O. Vretik, V. G. Syromyatnikov, V. V. Zagniy, E. A. Savchuk, O. V. Yaroshchuk, Mol. Cryst. Liq. Cryst. 2008, 486, 1099; (f) C. Kim, J. U. Wallace, S. H. Chen, Macromolecules 2008, 41, 3075; (g) Y. Yi, M. J. Farrow, E. Korblova, D. M. Walba, T. E. Furtak, Langmuir 2009, 25, 997; (h) S. Droge, M. O'Neill, A. Lobbert, S. P. Kitney, S. M. Kelly, P. Wei, D. W. Dong, J. Mater. Chem. 2009, 19, 274; J. L. Janning, Appl. Phys. Lett. 1972, 21, 173. Although these processes have demonstrated their durability and have been implemented in large-scale production environments, they usually require many fabrication steps, high processing temperatures, and sometimes, high vacuum environment.
In addition, many LC applications require patterned alignment of the LC to provide spatial modulation of the optical axis, for example, for the wave front control applications. Usually, in order to obtain patterned alignment, complicated and expensive photolithography techniques must be used. With the use of photoalignment, the process can be significantly simplified, but still requires design and fabrication of photo-masks as well as the deposition of a photosensitive polymer layer using spin coating and baking Other approaches include micropatterning using a sharp stylus, see G. P. Sinha, C. Rosenblatt, L. V. Mirantsev, Phys. Rev. E 2002, 65, 041718; (b) J. H. Kim, M. Yoneya, H. Yokoyama, Nature 2002, 420, 159, or micro-rubbing (μ-rubbing) of polyimides S. Varghese, S. Narayanankutty, C. W. M. Bastiaansen, G. P. Crawford, D. J. Broer, Adv. Mater. 2004, 18, 1600.
Another promising technique refined by Abbott and co-workers makes use of alkylthiol self-assembled monolayers (SAMs), see O. Guzman, N. L. Abbott, J. J. de Pablo, J. Chem. Phys. 2005, 122, 184711; (b) G. M. Koenig, M. V. Meli, J. S. Park, J. J. de Pablo, N. L. Abbott, Chem. Mater. 2007, 19, 1053; (c) V. K. Gupta, W. J. Miller, C. L. Pike, N. L. Abbott, Chem. Mater. 1996, 8, 1366; (d) R. A. Drawhorn, N. L. Abbott, J. Phys. Chem. 1995, 99, 16511, either on thin gold films sputtered on glass or gold islands immobilized on surfaces via electron beam evaporation, which, depending on the chain length, combination of chain lengths, and functionalization, can induce multiple alignment scenarios in nematic LCs, see (a) H. T. A. Wilderbeek, F. J. A. van der Meer, K. Feldman, D. J. Broer, C. M. W. Bastiaansen, Adv. Mater. 2002, 14, 655; (b) H. T. A. Wilderbeek, J. P. Teunissen, C. W. M. Bastiaansen, D. J. Broer, Adv. Mater. 2003, 15, 985. Photopatterning, S. D. Evans, H. Allinson, N. Boden, T. M. Flynn, J. R. Henderson, J. Phys. Chem. B 1997, 101, 2143, or microcontact printing (μCP) using SAMs pioneered by Whitesides, A. Kumar, G. M. Whitesides, Appl. Phys. Lett. 1993, 63, 2002, also allow for patterned alignment of nematic LCs, but still require multiple fabrication steps such as etching of a silicon wafer master to prepare PDMS stamps (that can be used many times for the same pattern, but not altered), see (a) J. P. Bramble, S. D. Evans, J. R. Henderson, C. Anquetil, D. J. Cleaver, N. J. Smith, Liq. Cryst. 2007, 34, 1059; (b) C. Anquetil-Deck, D. Cleaver, Phys. Rev. E 2010, 82, 031907.
In most cases, simple processes with fewer steps lead to lower production costs and higher yields. The development of simpler processes for the patterned alignment of LCs would facilitate the development of low-cost electro-optical devices such as adaptive LC-based lenses, see L. Li, L. Shi, D. Bryant, T. van Heugten, D. Duston, P. J. Bos, Proc. SPIE Optoelectronic Interconnects and Component Integration XI 2011, 7944, 79440S, or adaptive Bragg diffraction gratings, see C. C. Bowley, P. A. Kossyrev, G. P. Crawford, S. Faris, Appl. Phys. Lett. 2001, 79, 9.
The effect of homeotropic alignment of nematic LCs via doping with a small quantity of thiol-capped gold nanoparticles (NPs) has recently been demonstrated, see H. Qi, B Kinkead, T. Hegmann, Adv. Funct. Mater. 2008, 18, 212; H. Qi, T. Hegmann, ACS Appl. Mater. Interf. 2009, 1, 1731; and M. Urbanski, B. Kinkead, H. Qi, T. Hegmann, H.-S. Kitzerow, Nanoscale 2010, 2, 1118. The NPs migrate and adsorb to the interface formed between the LC films and the substrate, where they induce homeotropic alignment of the director over the entire area of the cell. A similar effect is achieved if NPs are deposited onto the surface before filling of the test cell with the LC material. This leads to a uniform coverage of the surface with the NPs and, in turn, uniform vertical alignment of the LC over the entire area. The homeotropic anchoring of the LC molecules on the NPs is accompanied by a contrast inversion effect, i.e. under the action of a low-frequency electric field, “dielectrically positive” LCs (Δε>0, the dielectric anisotropy Δε is defined as Δε=ε∥−ε⊥, where ε∥ is the dielectric permittivity parallel to the long molecular axis and ε⊥ the dielectric permittivity perpendicular to the long molecular axis) effectively act as dielectrically negative nematic LC (Δε<0) and undergoes a transition from the homeotropic to the homogenous state, see H. Qi, B. Kinkead, T. Hegmann, Adv. Funct. Mater. 2008, 18, 212. This dual-alignment capability can form the basis for numerous useful applications.